In general, a liquid developer for electrophotography is prepared by dispersing an inorganic or organic pigment or dye such as carbon black, nigrosine, phthalocyanine blue, etc., a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosine, synthetic rubber, etc., in a liquid having a high electric insulating property and a low dielectric constant, such as a petroleum aliphatic hydrocarbon, and further adding a polarity-controlling agent such as a metal soap, lecithin, linseed oil, a higher fatty acid, a vinyl pyrrolidone-containing polymer, etc. to the resulting dispersion.
In such a developer, the resin is dispersed in the form of insoluble latex grains having a grain size of from several .mu.m to several hundred .mu.m. In a conventional liquid developer, however, the soluble dispersion-stabilizing resin and the polarity-controlling agent are insufficiently bonded to the insoluble latex grains, so that the soluble dispersion-stabilizing resin and the polarity-controlling agent become freely dispersed in the liquid developer with ease. Accordingly, the soluble dispersion-stabilizing resin would be split off from the insoluble latex grains after storage of the liquid developer for a long period of time or after repeated use thereof, so that the grains would thereafter defectively precipitate, coagulate or accumulate, or the polarity would thereby become indistinct. Since the grains once coagulated and accumulated are reluctant to re-disperse, the grains would be adhered to everywhere in the developing machine, and, as a result, cause stain of images formed and malfunction of the developing machine such as clogging of the liquid-feeding pump.
In order to overcome such defects, a means of chemically bonding the soluble dispersion-stabilizing resin and the insoluble latex grains is disclosed in U.S. Pat. No. 3,990,980. However, the liquid developer disclosed was still insufficient, although the dispersion stability to spontaneous precipitation of the grains was improved in some degree. When the liquid developer was actually used in a developing apparatus, the toner adhered to parts of the apparatus and solidified to form a film thereon, and the thus solidified toner grains could hardly be redispersed. In addition, the solidified toner grains caused stain of the images duplicated and troubles in the apparatus. Accordingly, the liquid dispersion as disclosed in U.S. Pat. No. 3,990,980 was found to have a defect that the re-dispersion stability was still insufficient for practical use.
In accordance with the method of preparing the resin grains as disclosed in U.S. Pat. No. 3,990,980, there is an extreme limitation on the combination of the dispersing stabilizer to be used and the monomers to be insolubilized, in order to prepare monodispersed grains having a narrow grain size distribution. Mostly, the resin grains prepared by the method would contain a large amount of coarse grains having a broad grain size distribution, or would be polydispersed grains having two or more different mean grain sizes. In accordance with such a method, it is difficult to obtain monodispersed grains having a narrow grain size distribution and having a desired mean grain size, and the method often results in large grains having a grain size of 1 .mu.m or more, or extremely fine grains having a grain size of 0.1 .mu.m or less. In addition, the dispersion stabilizer to be used in the method has another problem in that it must be prepared by an extremely complicated process requiring a long reaction time.
In order to overcome the aforesaid defects, a method of forming insoluble dispersion resin grains of a copolymer from a monomer to be insolubilized and a monomer containing a long chain alkyl moiety, so as to improve the dispersibility, re-dispersibility and storage stability of the grains, has been disclosed in JP-A-60-179751 and JP-A-62-151868 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, a method of printing a large number of prints of 5000 or more prints has recently been developed, using an offset printing master plate by electrophotography. In particular, because of further improvement of the master plate, it has become possible to print 10,000 or more prints of large size by electrophotography. In addition, noticeable progress has been made in shortening the operation time in an electrophotomechanical system, and the step of development-fixation in the system has been conveniently accelerated.
The grains prepared by the methods disclosed in aforesaid JP-A-60-179751 and JP-A-61-151868 might be good in the mono-dispersibility, re-dispersibility, and storage stability of the grains, but showed unsatisfactory performance with respect to the printability for master plates of a large size and quickening of the fixation time.
Also, the dispersion resin grains prepared by the methods disclosed in aforesaid JP-A-60-185962 and JP-A-61-43757 were not always satisfactory in the points of the dispersibility and re-dispersibility of the grains and in the point of printability in the case of a shortened fixation time or in the case of master plates of a large size (e.g., A-3 size (297.times.420 mm.sup.2)) or larger.